vtec cuts out while driving

i have a gsr in my 99 si that i just recently bought. when i do a run down the street, the vtec will kick in when im in gears 1 and 2. but when i grab 3rd and around 5000 rpm the check engine light comes on and no vtec. after that the vtec wont come on at all.

when my head gasket went bad it did the exact same thing cuz the water n oil were mixing which made it lack the pressure to switch over. not saying thats the problem but just listing my experience of a road that ive been down. do u throw the cel which code did u get?

ive checked all of that but the screen. could a clogged screen do this. this is so frustrating what i have going on here.

i bought this car a couple weeks ago. its a 99 si with a 95 gsr motor. the ecu is a chipped p28. wtf why did he use a p28. he also has a jumper harness from the ecu to the main harness. i cant tell if the motor wiring is obd 1 or its the computer thats obd 1.are all 95 gsrs obd1

Update... i just talked to the previous owner and he informed me he bought the car in texas. he drove all the way back to colorado, where i live now. he said everything worked on the drive back until he got into higher elevation then the vtec was screwing up. he was told something about a baro sensor (atmospheric pressure sensor)